Ref: AI Tools/OpenAI/ChatGPT
Absolutely 👍 — here’s a ready-to-post, copyright-free blog article about how a DNS request moves across networks, including headers, ARP, and next-hop details.
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🌐 How a DNS Request Travels Across Networks
When you type a website name like www.example.com into your browser, your computer needs to find out the website’s IP address. This is where DNS (Domain Name System) comes in. But what really happens under the hood when your PC asks a DNS server for that IP address, especially when the server is outside your local network? Let’s break it down step by step.
1. Application Layer – The DNS Query
At the very top, your browser or operating system generates a DNS query.
- Protocol: DNS
- Port: UDP 53 (by default)
- Content: A small packet asking “What is the IP of www.example.com?”
2. Transport Layer – Wrapping in UDP
The DNS query is wrapped inside a UDP segment.
- Source Port: A random number (chosen by your computer)
- Destination Port: 53 (standard DNS port)
3. Network Layer – Adding IP Addresses
Next, the segment is wrapped in an IP packet.
- Source IP: Your computer’s IP (e.g.,
192.168.1.10) - Destination IP: The DNS server (e.g.,
8.8.8.8)
👉 Important: The destination IP stays the same all the way to the DNS server. It does not change as the packet moves across the internet.
4. Data Link Layer – Finding the Next Hop
Now we’re at the Ethernet/MAC layer. Your computer knows the DNS server is on a different network, so it must send the packet to its default gateway (your router).
But first, it needs the router’s MAC address, so it sends an ARP request:
- “Who has 192.168.1.1? Tell 192.168.1.10.”
The router replies with its MAC, and your PC builds the Ethernet frame:
- Source MAC: your PC’s NIC
- Destination MAC: your router’s NIC
- Payload: the IP packet destined for
8.8.8.8
5. Moving Through the Network
At each hop:
- The MAC addresses change (new source and destination for each hop).
- The IP addresses remain the same (your PC → DNS server).
Routers always check the IP header to decide where to send the packet next.
6. At the DNS Server
Finally, the packet arrives at the DNS server (8.8.8.8).
- The server sees destination IP = its own.
- It processes the query and sends back a DNS response.
- Source/Destination are reversed:
- Source IP =
8.8.8.8 - Destination IP = your computer
- Source IP =
✅ Key Takeaways
- Final destination IP never changes.
- MAC addresses change at every hop.
- ARP is only used inside local networks to find the MAC of the next hop.
- Routers forward based on IP, not MAC.
This is how a simple DNS request travels across networks — from your PC, through your router, across the internet, and back — all in just milliseconds.
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